Reproduction

Hydrozoans are mostly broadcast spawners. In some species only sperm is shed, and eggs are retained on the parent. Eggs release sperm-attracting compounds.

Mating System: polygynandrous (promiscuous)

Hydrozoan polyps reproduce asexually by budding, creating daughter polyps, medusae, or both. In some species medusae reproduce asexually as well, by fission or budding. Medusae (if present in the life cycle) or polyps produce gametes. Most hydrozoan species are dioecious, a few are sequential hermaphrodites. Eggs and sperm are most often released into the water column and fertilization is external. In some species eggs are retained and fertilized internally, in which case embryos may be releases as larvae or retained until even more developed.

Most hydrozoan species have minimal parental investment. Eggs and sperm are released into the water, and left to survive on their own. In a few species, eggs are retained in special structures on the parent, and the embryos are retained as brood, developing to the planula or even young polyp stage. In the latter case we have no information on whether the young are nourished by their parent, or just protected.